As we consider replacing the turtle hatch (see that thread here) it occurs to me that since we have access to amazing aluminum fabrication shops that making the new turtle hatch out of aluminum could be cheaper and lighter than steel, aluminum might be an option. The issue is one of insulating the steel and aluminum from one another.

To isolate the steel and aluminum it occurs to me that a single, shaped piece of 1/8 HDPE could be used between the turtle hatch edge and the deck, the bolt threads could be coated with lanocoat and we could use HDPE, or nylon washers or even neoprene compression washers to insulate the bolt heads.

I am keenly aware of the danger of mixing metals. An am curious for some feed back on this project.

Believe the SS United States is an example of mixed metals on a grand scale SS United States - Wikipedia, the free encyclopedia. The hull is steel and the surper structure aluminum. Above the waterline mixing metals shouldn't be a problem as long as you insulate the dissimilar metals from each other as you have outlined. Don't think you'd need 1/8" insulating gasket, just something thick enough to keep the metals from hooking up. Wouldn't want the insulating gasket to be continuous. Would want a way for any water that gets in to get out.

I think with careful thought the dissimilar metal issue is not a big deal. My aluminum boat certainly has a lot of stainless steel mounted to the decks, and so the same techniques should work in reverse.

Are you picky about the aesthetics? With a small thing like that it'd be fun to drop it off at a paint shop to be sprayed, but then you might end up with a perfect gleaming white that might not match the look of your decks. But if you paint it yourself, to match the decks, you might need an annoyingly small amount of aluminum paints.

The weight saving may not be significant if the steel version only weighs 40 pounds in the first place, but I do not know how much a turtle weighs.